<form [formGroup]="form">
  <mat-form-field>
    <mat-label>Label</mat-label>
    <input matInput formControlName="label" />
  </mat-form-field>
  <mat-list>
    <h3 matSubheader>Children</h3>
    <ng-container *ngFor="let child of form.children.controls | keyvalue">

    </ng-container>
    <mat-list-item (click)="form.children.add({ id: 'not-random' })">
      Add child
    </mat-list-item>
  </mat-list>
</form>